1. Field of the Invention
This invention relates to a lighting system using a light-emitting element.
2. Description of the Related Art
A light-emitting element is a self light-emitting element and is attempted to be used as a lighting system. The light-emitting element is a surface-emitting body light, and a lighting system capable of emitting near-natural light can be obtained by using the light-emitting element for lighting.
The light-emitting element has a layer containing a light-emitting substance which provides luminescence (Electroluminescence) by applying an electric field thereto, an anode, and a cathode. A hole injected from the anode is combined with an electron injected from the cathode in the layer containing a light-emitting substance, thereby obtaining luminescence. Luminescence obtained from the layer containing a light-emitting substance includes luminescence (fluorescence) that is obtained in returning from a singlet-excited state to a ground state and luminescence (phosphorescence) that is obtained in returning from a triplet-excited state to a ground state.
In a light-emitting device using such a light-emitting element, an electrode in the direction of light emission needs to be transparent. However, a transparent conductive film which is typically used as a transparent electrode often has relatively high resistivity, and voltage drop is caused in a portion away from a current supply terminal. In particular, the lighting system often made to emit light from the entire surface at the same luminance; therefore, in-plane nonuniformity of luminance becomes further noticeable.
However, when the lighting system has large area, luminance is lowered in a portion through which current is hard to flow. In other words, luminance is nonuniform in a light-emitting region of the lighting system. In addition, an ITO electrode typically used as the anode has higher electrical resistivity than that of metal such as Al used as the cathode. Therefore, voltage drop is caused in a portion away from a current supply terminal, which results in low luminance. In order to solve the problem, Reference 1 reports a structure in which at least a portion of an anode is provided with an auxiliary electrode having lower electrical resistivity than that of the anode (see Reference 1: Japanese Patent Laid Open No. 2004-134282).
In Reference 1, an auxiliary electrode is provided on a long side or a short side of a light-emitting element. However, when the lighting system has large area, luminance is lowered in a part away from the auxiliary electrode (for example, in a central portion of the lighting system). However, since light emitted from a light-emitting layer cannot be extracted outside when the auxiliary electrode is entirely provided over the anode in the structure of Reference 1, the auxiliary electrode can only be partially provided.
Since the lighting system is often made to emit light from the entire surface at the same luminance, nonuniformity of the luminance becomes further noticeable.